1. Field of the Invention
The present invention relates to a thin film transistor array substrate, a manufacturing method thereof, and a liquid crystal display device using the same.
2. Description of Related Art
A liquid crystal display device is one of thin panels, and widely used for monitors of personal computers and portable information terminal devices and TVs taking advantages including low power consumption and small and lightweight. The mainstream of current liquid crystal display devices is an active-matrix type including a plurality of display signal wires and a plurality of scanning signal wires arranged in a lattice pattern, and a thin film transistor (hereinafter referred to as TFT) formed as a switching element in a pixel region surrounded by the display signal wires and the scanning signal wires.
There is an increasing demand to a high-definition image display device by the development of multimedia industry.
The TN (Twisted Nematic) mode which is a conventional liquid crystal mode is a method to change a display state of on and off by applying a vertical electric field vertical to the substrate, raising and dropping liquid crystal molecules against a substrate surface according to an applied state of voltage. The TN mode does not have a good viewing angle characteristic due to its principle.
IPS (In-Plane Switching and IPS is a registered trademark of Hitachi Displays Ltd.) mode is a method to change the display state of on and off by applying a horizontal electric field substantially parallel to the substrate and moving liquid crystal molecules in plane which is substantially parallel to the substrate. The IPS mode has a wider viewing angle characteristic as compared to the TN mode, and is used for TV use etc.
In recent years, the FFS (Fringe Field Switching) mode, the improved IPS mode, has been developed. The FFS mode is a method to move the liquid crystal molecules by applying an oblique electric field not only the horizontal electric field, which is substantially parallel to the substrate, and has an excellent viewing angle characteristic in a similar manner as IPS.
In the IPS mode, both of a pixel electrode and a common electrode for driving liquid crystals are made to be a comb shape, and are arranged to interdigitate without the combs not touching each other. The IPS mode is a configuration in which the pixel electrode and the common electrode do not overlap in plane view. Usually, the gap between the pixel electrode and the common electrode in the IPS mode is larger than a cell gap and an electrode width.
In the IPS mode, the liquid crystal molecules that are positioned between the pixel electrode and the common electrode are driven in plane view, however the liquid crystal molecules that are positioned immediately above the pixel electrode and the common electrode are hardly driven. Therefore, the section immediately above the pixel electrode and the common electrode cannot contribute to display, and the transmittance is low, and thus it is difficult to obtain bright display properties.
On the other hand, in the FFS mode, the pixel electrode and the common electrode are disposed opposite with an insulation film interposed therebetween, and between the pixel electrode and the common electrode, a lower electrode does not include an opening, and an upper electrode is formed in a shape including an opening that generates a fringe electric field between the upper electrode and the lower electrode. The FFS mode is a configuration in which the pixel electrode and the common electrode overlap with an insulation film interposed therebetween in plane view.
For example, in the liquid crystal display device shown in FIG. 1 and FIG. 2 of International Patent Publication No. WO2001/018597, the upper electrode includes a plurality of slit-shaped openings in a plate-shaped electrode. In the liquid crystal display device shown in FIG. 3 of Japanese Unexamined Patent Application Publication No. 2001-056476 and FIG. 3 of Japanese Unexamined Patent Application Publication No. 2003-084303, the upper electrode has a comb structure.
The gap between the pixel electrode and the common electrode in the FFS mode is smaller than the cell gap and the electrode width.
In the FFS mode, the liquid crystal molecules positioned immediately above the electrode can be driven by using the oblique electric field. Therefore, when the pixel electrode and the common electrode are formed of a transparent conductive film such as an indium tin oxide (ITO), an electrode part can also contribute to display. Accordingly, in the FFS mode, higher transmittance can be achieved than the IPS mode.
In the liquid crystal display device of the FFS mode, unlike the TN mode and the IPS mode, two layers of transparent conductive films need be laminated as the pixel electrode and the common electrode with an insulation film interposed therebetween over the same substrate. These two layers of transparent conductive films only have a thin insulation film interposed therebetween, and a short-circuit between the electrodes is generated and a pixel defect may be generated.
In order to reduce the pixel defect due to the short-circuit between the electrodes, the insulation film disposed between the electrodes can be a multilayer structure, however it is not preferable as the number of manufacturing processes increases.